The invention concerns a ventilating device for a motor vehicle fuel tank, wherein a valve (12) is provided with a counter-weight (22) which, when the vehicle is not running in a substantially horizontal position, causes the valve (12) to be closed, and which, when the vehicle is running, causes the valve (12) to open. The invention is characterised in that the counter-weight (22) is supported on the upper concave surface (24) of a spring retainer (20) whereof one lower surface comprises means for closing the valve (12), and when the counter-weight (22) is spaced from its stable equilibrium position by vibrations, it makes the spring retainer (20) tilt so as to cause the valve (12) to be opened.